Bill of materials (BOM) and routings exist in different systems for different purposes. While an engineering bill of material is structured for engineering purposes its counterpart in manufacturing is structured to reflect manufacturing needs. A product, which both describe (as designed and as manufactured), is the same.
A routing usually only exists in manufacturing systems. However, the manufacturing process is often distributed over an enterprise resource planning (ERP) system and a manufacturing execution system (MES). While the routing in the ERP system needs to make sure that parts are available at the right time (production schedule) in the right location (work center) and collects material consumption data and cost, the routing in the MES is much more detailed, because it needs to describe precisely the manufacturing workflow, including loops, repairs, conditional branches and so on which are not relevant in the ERP system.
Since an ERP routing and a ME routing are so different (technically as well as content wise) in the ERP and ME system it is almost impossible to achieve a reasonable mapping between these two. This, in turn, makes it almost impossible to create a robust interface between ERP and ME system for routings. Even if the routing can be transferred from the ERP system to the ME system, the routing would need to be enhanced in the ME system and this step would need to be repeated every time a new version of the routing is created and transferred, which makes this process prone to errors.